def f1(a, L=[]):
    L.append(a)
    return L

def f2(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

print 'default is []'
print f1(1)
print f1(2)
print f1(3)

print 'default is None'
print f2(1)
print f2(2)
print f2(3)
